What does the train symbol represent in the Snowplow map?

You are looking at the train indicator, an element unique to the map "Snowplow".

Snowplow page on the Official Team Fortress 2 Wikia:

Snowplow is an Attack/Defense map which was intended to be released alongside the End of the Line Update, but wasn't included, as Valve deemed it "too confusing and challenging for new players"

The basic idea behind the map is that one team has to get the train into the other teams base, while the other team has to place obstacles to damage the train.

The train symbol is an indicator to the trains current health.

The % symbol means "percent". It derives from the English words "per centum", which was eventually shortened to "percent". It pretty much means "per 100". If you are at 100% health, your health is full. If it is at 50%, you have half of your total health. At 0%, your health is at 0.

If you are on the BLU team, you are on the offensive. Your objective is to get the train into the enemies base. By capturing control points, you will remove obstacles placed on the track, and move the train closer to the RED base.

Each time the train hits an obstacle, a timer will start. When the timer hits 0, a missile will be shot at the train, reducing its health by 10%. The timer will reset and continue to count down, until the control point has been captured, or the train has been destroyed. When the control point is captured, the train heals 10% of its health, and continues to the next obstacle.

If you are on the RED team, you are on the defensive. Your objective is to prevent the train from getting into your base, by preventing the BLU team from capturing the control points along the line.

If you find the map to be confusing, this is likely expected. As the quote reads, the map was held back; The main reason being that it was not "newbie friendly". That said, these sort of things grow on you, with experience.

You will find a more in-depth analysis of the map here, along with a better explanation of the game rules - including a run down of all the control point locations, and the multiplier conditions for adding time on capture.

Snowplow has a complex-appearing system of progress which is essentially decoration on a round timer for an attack/defend map. BLU's goal is to get the train to the end of the track, which it achieves by capturing control points. RED's goal is to destroy the train, which it achieves by preventing their capture until time runs out.

That's what the overall effect is. Here's how it comes about; the “lore”, if you will:

• The train has a health meter, which is the icon you see on your HUD. It starts each round with 100% health.

• The train proceeds rapidly along the track until it strikes an obstacle. There is one such obstacle for each control point; when the point is captured by BLU, the obstacle is removed and the train regains 10% health.

• At each control point, there is THE DEVICE (so called by the announcer) which is capable of damaging the train by 10%; it is located immediately above the stopped train and is some sort of dropped weight or ray-gun. THE DEVICE is DEPLOYED whenever the timer displayed at the top of your HUD (usually a round timer in other maps) reaches zero, and then resets for another hit.

• The timer is 34 seconds long, starts when the train hits an obstacle or after THE DEVICE has DEPLOYED, and is paused whenever the corresponding point is being captured. It is not possible for RED to directly attack the train or speed up the timer.

Thus, we effectively have a round timer of 340 seconds (plus train travel time), and BLU gains another 34 seconds when it captures a point if the health/timer is not already at max.

Irrelevant facts:

• The train can kill you. Do not stand in front of the train.

• THE DEVICE can kill you. Do not stand under THE DEVICE.